1. Field of the Invention
This invention relates to devices for recovering liquids and gas from wells, and more particularly to a plunger lift tool which aids in the recovery of liquids from low volume oil and gas wells.
2. Discussion of the Related Art
In many wells, both oil and gas are produced. Expensive traditional pumping systems are only cost justifiable when the well is flowing freely. Thus, prior to the development of plunger lift tools, many wells were simply abandoned when the flow rate decreased leaving substantial amounts of liquids and gas still in the well.
Known plunger lift tools are designed to inexpensively lift small amounts of liquid from such marginal wells. These tools use formation pressure to lift slugs of liquid to the surface in both deep and shallow wells.
A typical plunger lift tool is disclosed in U.S. Pat. No. 4,070,134. This device comprises a tubular outer shell, through which the gas passes during the descent of the device into the well. A velocity brake having a ball valve and restricting orifice is disposed in a nose portion of the tubular outer shell to slow the descent of the device into the well. When the device has reached a predetermined depth, a piston assembly closes a valve against a valve seat, thereby stopping the flow through the device. The pressure which builds up beneath the device then lifts the tool and the slug of oil above it to the top of the well. U.S. Pat. No. 2,999,545 discloses a well-known pressure relief mechanism. Additionally, U.S. Pat Nos. 1,836,871; 1,919,547; and 2,237,408 disclose various valve assemblies for use with plunger lift tools of this general type.
These prior devices suffer numerous practical disadvantages, however. The valve seat assemblies have proven incapable of developing adequate sealing force, and therefore, the valve would not stay shut. The valve seals also leaked. Additionally, the piston cylinder assembly of U.S. Pat. No. 4,070,134 had gas intrusion due to its position, thereby limiting its effectiveness in closing the ball valve at a predetermined depth. Moreover, the swab cup seals did not provide an adequate seal between the device and the casing.